The working machines are normally embedded with an operating device for operating and controlling an actuator. The operating device includes an operating member to be operated by an operator. The action of the actuator is controlled in response to an operation of the operating member. For example, a hydraulic excavator described in Japan Laid-open Patent Application Publication No. JP-A-2007-139146 includes a lower travelling unit, an upper revolving unit disposed on the lower travelling unit, and a revolving motor functioning as an actuator for revolving the upper revolving unit. The revolving motor is herein controlled in response to the operating direction and the operating amount of a lever of an operating device.
FIG. 5 represents the schematic configuration of the aforementioned operating device. In the operating device, either a first pilot pressure control valve 82 or a second pilot pressure control valve 83 is selected in response to the operating direction of an operating lever 81. The selected one of the first and second pilot pressure control valves 82 and 83 is configured to allow the hydraulic fluid to flow between a hydraulic fluid flow path 84 and a pilot hydraulic source 85, regulate the pressure of the hydraulic fluid from the pilot hydraulic source 85 in accordance with the operating amount of the operating lever 81, and output the regulated hydraulic fluid. Meanwhile, the other unselected one of the first and second pilot pressure control valves 82 and 83 is configured to allow the hydraulic fluid to flow between the hydraulic fluid flow path 84 and a tank 86. A pressure sensor 87 is configured to detect the hydraulic pressure in one of the operating flow paths 84, while a pressure sensor 88 is configured to detect the hydraulic pressure in the other of the operating flow paths 84. The hydraulic fluid flow paths 84 are herein connected through a restrictor 89. Further, a controller 90 is configured to control a revolving motor 91 based on the hydraulic pressures detected by the pressure sensors 88 and 88.
In the aforementioned operating device, the hydraulic fluid to be outputted from the first pilot pressure control valve 82 flows into the pressure sensor 87 through the hydraulic fluid flow path 84. So-called air entrapment may herein occur when the hydraulic fluid flow paths 84 are herein dead-ended at the pressure sensors 87 and 88. The air entrapment is a phenomenon that air contained in the hydraulic fluid resides in front of the pressure sensor 87. When air entrapment occurs, detection performance of the pressure sensor 87 may be deteriorated. In the aforementioned operating device, however, the hydraulic fluid flow paths 84 are connected through the restrictor 89. Further, when selected through the operation of the operating lever 81, the second pilot pressure control valve 83 is configured to connect the hydraulic fluid flow path 84 to the tank 86. The air, contained in the hydraulic fluid supplied to the hydraulic fluid flow path 84 from the first pilot pressure control valve 82, is thereby directed towards the tank 86 through the restrictor 89, the hydraulic fluid flow path 84 and the second pilot pressure control valve 83. When the second pilot pressure control valve 83 is selected, by contrast, the air, contained in the hydraulic fluid supplied to the hydraulic fluid flow path 84 from the second pilot pressure control valve 83, is directed towards the tank 86 through the restrictor 89, the hydraulic fluid flow path 84 and the first pilot pressure control valve 82.